x86/boot: Create a confined code area for startup code

In order to be able to have tight control over which code may execute
from the early 1:1 mapping of memory, but still link vmlinux as a single
executable, prefix all symbol references in startup code with __pi_, and
invoke it from outside using the __pi_ prefix.

Use objtool to check that no absolute symbol references are present in
the startup code, as these cannot be used from code running from the 1:1
mapping.

Note that this also requires disabling the latent-entropy GCC plugin, as
the global symbol references that it injects would require explicit
exports, and given that the startup code rarely executes more than once,
it is not a useful source of entropy anyway.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250828102202.1849035-43-ardb+git@google.com